Implantable cardiac stimulation devices are currently being used to treat various types of arrhythmias, such as ventricular tachycardia (VT) and ventricular fibrillation (VF). Such devices are capable of detecting the occurrence of an arrhythmia, and automatically applying an appropriate electrical stimulation or shock therapy to the heart aimed at terminating the detected arrhythmia. In addition to providing automatic stimulation, such devices often include a data acquisition system that is configured to acquire intracardiac electrogram (IEGM) signals, convert the raw analog data into digital signals, and store the digital signals for later processing and/or telemetric transmission to an external device.
Recently, there has been increased interest in adding myocardial ischemia detection capabilities to implantable cardiac devices. Myocardial ischemia, which involves oxygen starvation of the myocardium, can lead to myocardial infarction (MI) and/or the onset of malignant arrhythmias if the oxygen starvation is not alleviated. Although myocardial ischemia is sometimes associated with the symptom of angina pectoris (i.e., chest pain), many episodes of myocardial ischemia are asymptomatic or “silent.” The inclusion of myocardial ischemia detection capabilities within an implantable device can provide a physician with information about a patient's ischemic burden, which is especially useful when the patient is suffering from silent ischemia.
While trending of ongoing ischemia burden is of interest, it has so far been unclear how to make such information actionable by a physician for things other than detection of risk of an acute myocardial infarction (MI). In other words, it is still generally unclear when a patient's chronic ischemic condition has grown bad enough to warrant pharmacologic therapy, or an angioplasty or coronary artery bypass procedure. Currently, it is likely that the physician will have to rely as much or more on patient symptomatology as on implantable device diagnostics to make such a determination. However, if it could be demonstrated that transient ischemic episodes were precipitating an increase in potentially lethal arrhythmias, a physician might deem such information as sufficient reason to intervene, regardless of the presence or absence of symptoms such as chest pain. Accordingly, there is a desire to provide physicians with information that will allow them to readily identify correlations between ischemic episodes and arrhythmias.